Log SteamVR-started instances to logs/<exe>.log

The manifest passes --log-file, so a settings error (CFG-2) ends up in a
log file in the install folder. Package README and docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PROGRESS.md

@@ -8,9 +8,9 @@ CLAUDE.md ("Working rules").
 - **Implementation iteration 1 (DSH-1 and configuration), phase 3 (implementation) done,
   waiting for the user's approval.** All plan steps are implemented and committed; next is
   phase 4, testing on the Frame (plan item 10).
-- **Open point found:** started by SteamVR, the app writes no log file (the manifest passes no
-  `--log-file`, stderr goes to SteamVR). CFG-2 wants the settings error "in the log file and
-  on stderr". To be decided.
+- Started by SteamVR, the app now logs to `logs/frame-assistant.log` in the install folder:
+  the manifest passes `--log-file` (CFG-2 wants the settings error in a log file). Whether
+  SteamVR on the Frame honours the manifest's `arguments` is checked in phase 4.
 
 ## Next steps
 
@@ -95,6 +95,9 @@ Known limitation: the bundled-Qt package does not support WebEngine in this iter
     generated QML type registration includes headers by file name; Qt ≥ 6.9 hints at
     `WebEngineProfilePrototype` (not in the Frame's 6.8). Deviations: `HaWebUi` lives in the
     app, not the library; the password warning needs a password in the file.
+  - Log file for SteamVR-started instances: the manifest passes `--log-file
+    logs/frame-assistant.log` (user's choice among: default log path in the app, manifest
+    argument, document only).
   - Not verified yet: WebEngine with a URL in the off-screen pipeline (Frame, phase 4); the
     bundled package (not supported for WebEngine in this iteration).
 - Plan for implementation iteration 1 approved; phase 3 started.